Mobile communication networks enable wireless communication to and from a mobile user equipment. Typically, the user terminal is served by a base station within an area and the base station is coupled to other network elements within the network. The connection of the base station to the other network elements is usually implemented by wired means in order to provide secure channels with high capacity for communication. As the mobile communication is based on the fundamental thinking that the user equipment may move from one place to another within the network coverage a reasonable amount of control and management relating to the mobility is needed.
Mobile communication network can be divided to two main entities. First one is so called core network (CN), which is a central part of the network providing various services to customers who are connected by the access network. The second entity is called radio access network (RAN), which implements a radio access technology for user equipments within the coverage of the network. The radio access network typically comprises base stations and possibly some controlling elements for the base stations. The radio access network is controlled and managed at least partly by the core network.
Mobile communication networks normally comprise a network element residing typically in the core network, which is mainly taking care of the control and management functions, such as end-to-end connection, mobility and handover related tasks. In GSM and UMTS such an element is a Mobile Switching Centre (MSC). In Long Term Evolution (LTE) network similar element is Mobility Management Entity (MME). Said network elements are normally specific to different location areas i.e. one network element takes care of the control and management functions within a location area.
In addition to a network element taking care of the control and management functions the core network comprises at least some registers in order to store and maintain subscriber related information. First such register is a so called Home Location Register (HLR), which is a database storing permanently information on subscribers. More specifically, HLR stores details of Subscriber Identity Module (SIM) cards issued by a telecom operator. Each SIM has a unique identifier called International Mobile Subscriber Identity (IMSI), which is primary identifier for each record stored in the HLR. The HLR also stores e.g. Mobile Subscriber Integrated Services Digital Network Number (MSISDN), which is a number uniquely identifying a subscription in a mobile communication network and used of routing calls to the subscriber. Other pieces of information stored in the HLR are location information of the UE and roaming related issues among others. Even if it is said that there is only one HLR for any given network, in practice the information stored in the HLR is spread out across the network into multiple physical HLRs.
Second register type belonging to core network is so called Visitor Location Register (VLR). VLR is a database containing at least partly the same information as stored in the HLR. The difference is that when the HLR contains the information on all the subscribers for the telecom operator, the VLR contains information on subscribers currently residing in its location area. This is consequence of the implementation of the mobile communication network in which the network is divided to different location areas comprising necessary network elements in order to provide the service for the subscribers residing in that specific area and reducing thus the load to the central network elements and registers, such as VLR, as well as reducing the network traffic. Different VLRs can be identified with a Location Area Code (LAC) for the area they serve. A typical way to implement a VLR is to integrate it to a network element taking care of a management and controlling functions, such as MSC.
FIG. 1 illustrates a simplified network structure, which clarifies the hierarchy of the different network elements and devices as described above. The FIG. 1 illustrates two location areas A and B. Both location areas A and B comprise a network element 101A and 101B for control and management functions, which are configured to interact with each other e.g. in a situation when a user equipment moves from a first location area A, B to the other location area A, B. The network element 101A, 101B is configured to control and manage the radio access network. FIG. 1 illustrates only the base stations 102A, 102B belonging to the corresponding radio access network. The radio access network may comprise some further elements for some further purposes, such as further controlling and management, which are not illustrated here. The base stations 102A, 102B are configured to communicate with the user terminals 103A, 103B allocated within the location area A, B. Furthermore, the network elements 101A, 101B are configured to communicate with subscriber registers 104A, 104B, such as VLRs, belonging serving the same location area as the network element 102A, 102B. The subscriber registers 104A, 104B serving the location area are configured to receive and transmit information to and from central subscriber register 105, such as a HLR. Furthermore, the network elements 101A, 101B may communicate directly with the central subscriber register 105 (not illustrated in FIG. 1). The network element 101A, 101B can be e.g. MSC or MME depending on the implemented radio communication technology. Similarly, base stations 102A, 102B may implement GSM or 3G based radio technologies or LTE technology, in which case the base stations area called eNodeBs.
Each subscriber shall not be present in more than one VLR 104A, 104B at a time. However, for some reasons it may happen that the same user equipment (UE) 103A, 103B is registered to two VLRs 104A, 104B simultaneously which, in turn, causes call attempts toward the UE 103A, 103B to fail until the dual VLR 104A, 104B registration is canceled. More specifically, when UE 103A, 103B is registered in two VLRs 104A, 104B, one VLR 104A, 104B is known by the HLR 105 in the mobile network, and terminated calls are directed to that VLR 104A, 104B. The other VLR 104A, 104B is handling the UE 103A, 103B, and UE originated calls are handled by that VLR 104A, 104B. The former VLR 104A, 104B is the wrong VLR 104A, 104B, and the latter is the correct VLR 104A, 104B.
Being registered in two VLRs 104A, 104B is an error case that is caused by cancel location messages being lost between HLR 105 and VLRs 104A, 104B. The cancel location messages can be lost if there is a disturbance in the mobile communication network so that the VLR 104A, 104B is not reachable by any other network element. Then no messages will be received by the VLR 104A, 104B, and an active UE will also be assigned to other VLRs 104A, 104B. When the disturbance in the communication network ceases, dual VLR registration may exist for some UEs.
Today the solution to correct the dual VLR 104A, 104B situation is to wait customer complaints and in response to that reset the VLR 104A, 104B. This makes the UE to register again to the mobile communication network and a correct VLR 104A, 104B can be allocated. However, resetting VLRs 104A, 104B is a costly process causing disturbance to all subscribers allocated to the VLR 104A, 104B to be reset i.e. subscribers are not able to receive calls during the downtime of the VLRs 104A, 104B.
The above described situation may especially cause problems in the current telecommunication network setup in which multiple networks with different technologies are available in a certain location. For example, one telecom operator may have a LTE, WCDMA and GSM networks available. As it is known that LTE network does not currently offer any circuit switched services i.e. in case of a subscriber is served by the LTE network and he or she receives a mobile terminated call, the subscriber needs to be transferred to either WCDMA or GSM network in order to establish the call connection. Such an operation between the network technologies is called circuit switched fallback (CSFB). If the subscriber in such a case is allocated to multiple VLRs, it is probable that the subscriber cannot be transferred into a circuit switched network without any problem and the establishment of a call connection fails. Thus, it is important that there exists only one VLR association for one user terminal in a LTE network at a time.